1. Field of the Invention
The present invention relates to a magnetoresistive head (to be referred to simply as "MR head" hereinafter) especially for reading magnetic information written in a magnetic recording medium using a spin-valve effect.
2. Description of the Related Art
As recording densities in disk drives increase, an MR head having a high reading sensitivity has been put to practical use as a magnetic head adapted to writing and reading magnetic information with a narrow track width and high linear density. For advanced magnetic disk drives with higher recording densities, attention has been paid to an MR head using a Giant Magnetoresistive (to be referred to as "GMR" hereinafter) effect as a magnetic head having much larger sensitivity.
The GMR effect has been observed in a variety of magnetic multi-layered structures, the essential feature being at least two ferromagnetic metal layers separated by a nonferromagnetic metal layer. The physical origin is as follows: the application of an external magnetic field causes a variation in the relative orientation of the magnetizations of neighboring ferromagnetic layers. This in turn causes a change in the spin-dependent scattering of conduction electrons and thus the electrical resistance of the structure. The resistance of the structure thus changes as the relative alignment of the magnetizations of the ferromagnetic layers changes.
A particularly useful application of GMR is a spin valve structure device as disclosed in Japanese Unexamined Patent Application No.4-358310. It consists of a ferromagnetic free layer and a ferromagnetic pinned layer separated from each other by a thin spacer layer. The magnetic moment of the pinned layer is typically fixed in a direction perpendicular to the magnetic moment of the free layer under conditions of no signal field, by exchange coupling with an adjacent antiferromagnetic layer, with the magnetic moment of the free layer allowed to rotate in response to signal fields.
The resultant spin-valve response (i.e., the change in resistance) is proportional to the cosine of the angle between the direction of the magnetization in the two magnetic layers (i.e., the free layer and the pinned layer). The change in resistance of this is then proportional to sin .theta., where .theta. is the angle of the magnetization in the free layer with respect to the longitudinal axis of the device.
It is essential for the spin-valve device of this structure, whether or not an anti-ferromagnetic layer, to enable the magnetization of the pinned layer to be completely fixed against disturbance. It is, therefore, an important challenge to select an anti-ferromagnetic layer capable of obtaining a sufficient exchange field in a stable manner.
To avoid the difficulty mentioned above, there is proposed a spin-valve device which dispenses with an additional structural means to fix the magnetization direction of a "pinned layer" as disclosed in Japanese Unexamined Patent Application No. 6-203340. FIGS. 4A through 4C illustrate the spin-valve device thereof.
As shown in FIGS. 4A through 4C, a reading head unit comprises a rectangular spin-valve device unit 119 having an underlayer 121, the first layer 122 of a ferromagnetic material, a non-magnetic metal layer 123, and a second ferromagnetic layer 124 sequentially formed above a substrate 120. The spin-valve device 119 also comprises electrical lead layers 126 and 127 provided on the upper surface of both end portions of the spin-valve device unit 119 of FIG. 4A and a bias conductor 128 (see FIG. 4A) for generating a magnetic field h parallel to the magnetic easy axes 130 of the ferromagnetic layers 122 and 124 of the spin-valve device unit 119. Electrical leads are provided to form a circuit path between the MR sensor and a current source 34 and a signal sensing means 33. In order to reduce Barkhausen noise, a longitudinal bias layer 125 is deposited over one end of the MR element remote from the ABS of the sensor.
The magnetic easy axes 130 of the above-stated ferromagnetic layers 122 and 124 are aligned along the vertical axis of the ferromagnetic layers 122 and 124 and perpendicular to the data track width w of a magnetic storage medium, respectively.
When sense current 129 is applied to the spin-valve device 119 through the electrical lead layers 126 and 127, a magnetic field is generated by the sense current 129 and the magnetic moments 131 and 132 of the ferromagnetic layers 122 and 124 are set to rotate from the easy axes 130. The rotation angles have the same value a with the opposite directions, respectively.
Both magnetic moments 131 and 132 are allowed to rotate freely in response to signal fields.
If a signal field having a sufficient magnitude is applied to the longitudinal direction of the spin-valve device 119 from a magnetic storage medium (not shown), then the magnetic moments 131 and 132 of both of the ferromagnetic layers 122 and 124 rotate at angles almost equal in magnitude and opposite in direction from the magnetic easy axes 130, respectively.
In the prior art of FIG. 4, therefore, unlike a type of a device in which the magnetization of one of the ferromagnetic layers is fixed, an additional means for fixing the magnetization direction of the "pinned layer" is not required. Besides, since the angle of magnetization direction varies two-fold, a higher output (resistive change) can be, in principle, obtained in proportion to the cosine of the angle.
In the above-stated prior-art, however, the spin-valve device 119 is provided in a longitudinal (or vertical) configuration perpendicular to the magnetic storage medium. Due to this, the characteristics according to the principle cannot be obtained in practice.
The reason is as follows. As well known, the intensity of a signal field from a magnetic storage medium attenuates (or is in inverse proportion to the square of a distance from the magnetic storage medium) as the distance become longer. As a result, the signal field with a sufficient magnitude cannot be obtained except in the neighborhood of the ABS (or Air-Bearing Surface relative to the magnetic storage medium) of the vertical-type device.
According to the prior art, the magnetization of the spin-valve device 119 rotates only in the neighborhood of the ABS and resistivity changes as well as outputs are therefore quite slight.
Although the prior art mentioned above do not disclose, the spin-valve device 119 is arranged between a pair of magnetic shield layers while predetermined insulating layers are usually provided between the device 119 and the magnetic shield layers, respectively to improve reading field resolution.
Furthermore, as the recording density increases, the recording density (or linear density) of the traveling direction of a magnetic storage medium increases. To efficiently read the signal field recorded at high linear density from the magnetic storage medium, the distance (or gap) between the spin-valve device 119 and each of the magnetic shield layers needs to be narrowed.
In the prior art stated above, the electrical lead layer 126 for applying sense current needs to be provided on the ABS side. For that reason, the distance or gap needs to be at least equal to the thickness of the electrical lead layer 126 and cannot be further narrowed.